A well-known "line-doubling" technique employs the conversion of an interlaced television signal (such as employed in the NTSC, PAL and SECAM systems), in which two time consecutive interleaved fields of scan lines represent a frame, to a non-interlaced or progressively scanned television signal, in which all of the scan lines in the frame are scanned out sequentially. The result, in the case of NTSC, is that 525 scan lines are presented every one-sixtieth of a second rather than 262.5 scan lines, thus potentially doubling the vertical resolution to the full frame resolution of 525 lines. Typically, line doublers employ, at least part of the time, field merging or inter-field interpolation, which provides a full doubling of the vertical resolution. Pairs of fields are combined to form a single progressively scanned frame. Such arrangements are described in U.S. Pat. Nos. 4,876,596; 4,967,271; 4,982,280; 4,989,090 and 5,159,451, each of which hereby is incorporated by reference in its entirety. Such an arrangement is also described in allowed U.S. Pat. No. 5,291,280, which is hereby incorporated by reference in its entirety.
One problem resulting from field merging is that there is a time difference between fields in the interlaced television signal. Thus, a moving object may be in different locations from field to field and, if two such fields are combined to form a single frame, the vertical and diagonal edges of objects moving horizontally or diagonally appear as ragged sawtooths rather than smooth curves. Line doubling is lost along such transitions. The sawtooth effect is exacerbated if the reproduction system employs vertical enhancement.
One solution to this problem is to employ some type of scan line or intra-field interpolation in regions of the television picture display in which movement is occurring. An improved technique for providing intra-field interpolation in areas of the picture having motion is set forth in said U.S. Pat. No. 4,989,090.
A potential problem with switchable inter-field/intra-field interpolation line doublers is that motion may not be properly detected at all times, thus allowing the above mentioned jagged vertical or diagonal transition artifacts to appear briefly and intermittently. Such arrangements may have particular difficulty in consistently switching correctly between operating regimes when operating on an interlaced television signal that has been subject to data compression or standards conversion (as from PAL to NTSC, for example).
Another known solution to the problem is to recognize when the source of the interlaced television signal is a motion picture film. In the United States, 24 frame per second motion picture films are applied to the 30 frame per second NTSC television signal by means of a "3-2 pulldown ratio," such that a patterned sequence of 3 film frames, 2 film frames, 3 film frames, etc. are carded by consecutive television fields. By identifying pairs of interlaced fields carrying the same film frame, the pairs may be merged to generate essentially perfect line doubled progressively scanned frames, each representing a motion picture frame. Such techniques are described in said U.S. Pat. No. 4,876,596 and 4,982,280.
However, the motion picture film detection arrangement is also subject to problems. For example, a video edit (rather than a film edit) may disrupt the 3-2 pulldown ratio, causing merged television fields to be derived from different film frames. Thus, until the film pulldown ratio detection circuitry resynchronizes itself (typically one to four fields of the input interlaced television signal), the ragged sawtooth transition artifacts, described above, may appear at the edges of moving objects. Although the undesired effect is fleeting, it is perceptible to the eye, occasionally disturbing an otherwise near perfect line doubling arrangement.
The last discussed problem may also arise in the motion picture film source line doubling arrangement when the input interlaced television signal fades from a film source to a video source. In that case, the system, as described, for example, in said U.S. Pat. No. 4,982,280, continues to operate in its "film mode" for a few fields before recognizing the loss of the 3-2 pull down pattern and switching to an inter field/intra-field interpolation arrangement of the sort described above. During those few fields before changeover from the film mode of operation, the same sort of ragged sawtooth artifacts may be generated if there is motion between the fields. In addition, the problem may also arise when the interlaced television signal derived from a motion picture source has been imperfectly data compressed or standards converted.
One solution to the problem of video edits disrupting the 3-2 pulldown ratio is set forth in said allowed U.S. Pat. No. 5,291,280, of applicant Yves C. Faroudja. The solution requires the use of a field delay and relatively complex circuitry in order to "look ahead" for video edits. Such a solution adds a further one field delay to the four field delay which is typically employed in line doubling progressive scan converters, resulting in a perceptible delay between the picture and undelayed audio. Thus, such an arrangement requires the use of a high quality audio delay, adding further cost and complexity.